In modern transmitter arrangements, complex signal processing is widely used. A complex signal can be represented by an in-phase component I and a quadrature component Q. This is normally referred to as Cartesian representation. I and Q signals are separated by a phase of 90°. However, when applied in a transmitter in the digital communications field, a conventional modulator needs two carrier signals, namely of 90° phase difference but at the same frequency. These multiplied signals are summed together and passed to the output driver. A resulting signal has a continually variable amplitude and phase.
However, a complex signal can also be represented by amplitude and phase. This is also referred to as polar representation. In a polar modulator, the amplitude can directly be used to control the output amplitude, typically by converting the amplitude to an analogue value which drives a mixer. Phase information can be used to control a frequency synthesizer such that a phase of a generated carrier signal tracks the desired phase.
Polar modulation offers a potential for reduced complexity and current consumption in the modulator path, removing the requirement to generate separate I and Q carrier signals, eliminating the problem of image rejection and making frequency synthesis more robust by allowing the oscillator to operate at the same frequency as the carrier frequency or an integer multiple thereof. Moreover, since more and more functionality can be implemented in the digital domain instead of the analogue domain, the polar modulation concept is more suitable for implementation in advanced, complementary metal oxide semiconductor (CMOS) processing technologies.